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David Brin (a great raconteur as well as an amazing writer—never pass up a chance to hear him speak) pegged the economic aspect of the issue: there is never a colony without an economic interest to sustain it. But, I’ve been to law school, where I was taught not to “fight the hypo,” meaning that when someone gives you a hypothetical situation, you accept it as given and go from there because the terms of the hypo are designed to spur thinking on a particular issue.

So, I’ll simply assume for the purposes of the discussion that there is some valid reason to colonize the moon, and a political desire to do so, and—accordingly—the necessary funding and other resources. Which, I think, is what the original question was getting at.

Given the foregoing, it is important to understand that, as of 1972, you have the following issues to solve in order to get a lunar colony going.

As Michael Ross has pointed out (in his autodidactic and iconoclastic way!), we’ve still got a payload problem. You’ve got to push a lot of weight out of the Earth’s gravity well by brute force, let it be grabbed by the moon’s gravity, and then exert some more—but significantly less—force to slow it down enough that when it gets to the moon it doesn’t just make another crater. This problem isn’t going to go away unless and until there is some sort of enormous breakthrough in basic science—something like a quantum theory of gravity—that lets us get off the Earth without having to burn a lot of rocket fuel to do it. That ain’t happening soon.

Colonies must be self sufficient in terms of consumables, or at least close enough to self sufficiency that they do not require constant supply from home of basic needs. The problem here is that even under the harshest conditions on Earth, colonies didn’t have to do anything special to have enough air to breath or (with few exceptions) water to drink. But, as hard as it was for the Pilgrims to get goods from England, getting water and air and food from Earth would be even more difficult for moon colonists. A lunar colony (as opposed to a scientific station or outpost, which is a different proposition entirely) would need to be able to meet all or virtually all of its needs for air and water, and the overwhelming majority of its food requirements—otherwise the cost would be beyond prohibitive. No conceivable economic interest would be enough to pay for it. This is not an impossibility by any means—there is probably ice in large quantities in craters near the poles and it would not surprise some selenologists to find other sources as well; oxygen can be made from water or there are processes for extracting it from chemicals in the lunar soil; lunar soil can be turned into a growth medium for plants, although artificial light is probably necessary because the 14 days of day/14 days of night probably won’t work for any crops we know of. But, there are lots of details to work out in order to be able to do all of these things practically and with equipment that could be easily transported to or assembled on the moon.

Radiation is a major issue if you are going to have people actually living on the moon. It’s one thing to stay there a few days as our astronauts did in Apollo, but people who live there permanently are going to need some serious radiation protection, which means that they are probably going to have to live underground or build structures on the surface and cover them with a foot or two of regolith. The Apollo missions left radiation detectors behind that allowed us to monitor radiation on the moon for a few years, so we have a good idea of what we’re up against.

None of these issues is something that can’t be overcome or managed, but the practical aspects of providing consumables for the colonists would require a sustained and expensive engineering effort followed by the construction and testing of prototypes on the moon before real colonists relied on them for air and water and food. Given, however, the rate at which we solved the problems associated with Apollo, and with ample funding and by making it a high national priority, ten to fifteen years from when Gene Cernan and Jack Schmitt took off from the Taurus-Littrow valley on 14 December 1972 hits me as a reasonable time frame.

This is only a somewhat educated opinion and there is a large range of reasonable/arguable responses. I wouldn’t argue with anyone who said that it could be done in seven years or with anyone who said it would take twenty-five.